In the first part of this series, featured in the February 2011 issue, we considered the selection of the protective devices of a low-voltage induction motor supplied from a motor control center (MCC). The second part (March 2011) examined the phase overcurrent settings of the feeder breaker that serves the MCC. In this third and final part of the series on the protective device coordination procedure for a typical commercial and industrial power system, phase overcurrent and ground-fault protection considerations are addressed for the primary-side fused switch, transformer, and secondary-side main breaker where the MCC feeder breaker resides.

As with the other parts of the series, which are all based on the 2008 NEC, discussion of this topic is brief and incomplete, given the fact that other crucial application-specific considerations are not addressed. It goes without saying that the electrical designer on any particular project must work closely with the client and application engineers of manufacturers to select the electrical equipment best suited for the application.

Phase overcurrent protection

Figure 1(click here to see Fig. 1) provides the one-line diagram and background information to address the phase overcurrent protection considerations of the primary fuses, transformer, and secondary main breaker of the substation. Note: The phase overcurrent settings of the MCC feeder breaker were determined in Part 2 of this three-part series — and are provided here for reference. Figure 2(click here to see Fig. 2) presents the time-current plot for this situation. The following discussion is intended to support the results.

We must first discuss the phase overcurrent protection considerations of the secondary main breaker. Referring to Fig. 2, the overall time-current characteristic of the secondary main breaker meets the necessary requirement of being completely coordinated with the transformer through-fault protection curve. Furthermore, its long-time pickup or current setting (CS) of 0.6 × 1,600A rating plug = 960A meets the limit in NEC Table 450.3(A) of 1.25 × transformer full-load amps (FLA) = 1,127.5A, and the minimum (left-most) edge of the long-time tolerance band does not encroach on the full-load capability of the transformer (902A).

Although the trip unit is equipped with an instantaneous function, it poses no coordination issue with the MCC feeder breaker. This is because we were able to set the instantaneous pickup level (not shown in Fig. 2) above the maximum fault current of 19,000A at 15 × 1,600A rating plug = 24,000A. The short-time settings enable the main breaker to interrupt low-level faults at the main breaker (of magnitude greater than about 5,000A) before exceeding the short-circuit time duration limit (30 cycles or 0.5 sec) of the switchgear. Furthermore, the short-time settings are insensitive to the maximum load amperes (MLA) of the substation, beginning at 545A (motor starting amps of largest 50 hp motor of MCC) + 430A (remaining load of MCC) + 300A (remaining load of substation) = 1,275A and decaying to 363A (locked rotor amps of largest 50 hp motor) + 430A + 300A = 1,093A by 0.1 sec.

Next, we address the phase overcurrent protection considerations of the primary fuses. As shown in Fig. 2, the characteristic of the 40E (full-range) primary fuses complies with the NEC Table 450.3(A) limit of 3 × transformer FLA = 2,706A at 480V (94A at 13.8kV), clears the transformer inrush point, conservatively set at 12 3 transformer FLA = 10,824A at 0.1 sec, and can interrupt the maximum fault current at the main breaker (19,000A) well before the short circuit time duration limit (30 cycles or 0.5 sec) of the switchgear. However, due to the NEC and inrush constraints, the characteristic does not completely coordinate with the transformer through-fault protection curve, so the transformer is vulnerable to damage from low-level faults that could occur anywhere between the load side of the fuses and the line side of the main breaker.

Finally, it must be understood that the preceding observations are specific to the rating and characteristic of the E-rated (full-range) fuse in this example — and the rating could be different for a different fuse type.

Ground-fault protection

Figure 3(click here to see Fig. 3) provides the one-line diagram and background information to address the ground-fault protection considerations of the secondary main breaker and MCC feeder breaker of the substation.

The trip unit of the secondary main breaker must include the ground-fault function, per NEC 215.10 for feeder disconnecting means or 230.95 for service disconnecting means. The maximum possible pickup and delay settings for this trip unit of 0.6 × 1,600A sensor = 96 A and max / I2t out, respectively, were chosen; and these comply with the requirements of 230.95(A).

The trip unit of the MCC feeder breaker includes the ground-fault function, although it is required only in health care facilities per NEC 517.17(B) and critical operations power systems per NEC 708.52(B). The maximum possible pickup setting and delay settings that were chosen are 0.6 × 800A sensor = 480A and int / I2t out, respectively.

In Fig. 4, (click here to see Fig. 4) the time-current characteristics that correspond to the ground-fault function settings of the breakers are superimposed on those of the phase overcurrent protective devices. The following discussion examines how they respond to ground faults at key locations of this system.

The arcing (minimum) and bolted (maximum) ground-fault currents at the 50-hp motor are 760A and 2,000A, respectively. For the MCC feeder breaker ground-fault function pickup and delay settings cited above, notice in Fig. 4 that the 90A fuse (not the MCC feeder breaker) will rightfully interrupt a ground fault at the motor. This conclusion can be extended for a ground-fault anywhere downstream of the 50-hp motor starter fuses, since the arcing and bolted ground-fault currents will be higher than those at the motor.

The arcing and bolted ground-fault currents at the line side bus of the MCC are 1,330A and 3,500A, respectively. Given the secondary main breaker ground-fault function delay settings cited above, the MCC feeder breaker (not the secondary main breaker) will rightfully interrupt a ground fault at the line side bus of the MCC. This conclusion can be extended for a ground fault anywhere else along the MCC feeder, although the feeder breaker will trip on instantaneous for fault currents above about 3,800A.

The arcing and bolted ground-fault currents sensed by the secondary main breaker for a ground fault on the load side of the main breaker at the substation switchgear bus are 4,750A and 12,500A, respectively. This fault will be interrupted by the secondary main breaker on ground fault from 4,750A to about 6,300A. From about 6,300A to 12,500A, the short-time function directs the secondary main breaker to interrupt the fault. The primary fuses do not respond to this fault.

For a ground fault anywhere from the secondary of the transformer to the line side of the secondary main breaker, we must rely on the primary fuses to interrupt the fault. The arcing and bolted ground-fault currents sensed by the primary fuses are 4,750A and 12,500A at 480V, respectively; and the corresponding worst-case times to clear such values of currents are 10 sec and 0.25 sec.

Finally, for a ground fault anywhere from the load side of the primary fuses to the primary of the transformer, the arcing and bolted ground-fault currents sensed by the primary fuses are 423A and 1,113A at 13.8kV or 12,160A and 32,000A at 480V, respectively; and the corresponding worst-case times to clear such values of current are 0.28 sec and 0.025 sec.

In summary

This article has considered the requirements to select the ratings, characteristics, and settings of the phase overcurrent and ground-fault protective devices of a low-voltage unit substation. Due to space limitations, this presentation is brief; therefore, the reader should refer to IEEE Std 242-2001 (IEEE Buff Book) for further understanding of this important topic.

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